Injection molding processes commonly involve the use of two or more dies which fit together to define at least one enclosed space. This space or cavity has a shape of the article to be molded. When small articles are manufactured there are usually several cavities in a single mold which are connected by passageways or runners through which molten plastic can flow into the mold cavities. Plastic material is injected into the mold and runs along the runners in the mold cavities. Sometimes gates are provided through which plastic flows into and fills the cavities. After the molded resin has cooled sufficiently, the mold halves are separated and the articles are removed from the cavities. Normally, plastic has remained in the runners and cooled to form arms which interconnect the molded workpieces. As the molded articles are removed from the cavities they remain connected by the plastic arms. They must then be broken away from these arms. The separation process is a manual operation done either at the factory or by the purchaser. When such manual separation is a part of the manufacturing process it increases the costs of manufacture. Another problem that may occur during this separation process is that a tear or ragged projection may form in the article at the broken connection between an arm and the workpiece. These defects can only be manually removed.
Suction cups conventionally have a convex surface which is placed against a flat surface to which the suction cup is to be attached. The suction cup is sufficiently pliable so that it can be pressed against the flat surface. This causes the concave surface to deform and flatten against the flat surface. Since the suction cup is made of a resilient material, it attempts to return to its original shape. This creates a small vacuum between the suction cup and flat surface; the vacuum holds the suction cup in place. Any depressions which may exist in the concave surface will cause air pockets to be formed between the suction cup and flat surface when the suction cup has been pressed against it. These air pockets reduce the adhesion between the suction cup and flat surface. Then, less force is required to remove the suction cup from the flat surface. This means that a suction cup having such irregularities in its concave surface will not hold as well.
It is well-known that plastic shrinks when it cools. Thus, one must anticipate some shrinkage in the molding of any plastic product. The prior art has manufactured suction cups by injection molding in which the plastic was injected through gates adjacent to the top surface of the cup portion of the suction cup. I have found that when suction cups are molded in this way a depression will occur in the concave surface. This depression reduces the holding ability of the suction cup for the reasons just described. Consequently, there is a need for an injection molding process for suction cups which compensates for plastic shrinkage without reducing the holding power of the suction cup.
By injecting plastic into the top or head of the suction cup, one could avoid the problems caused by injecting plastic into the cup portion of the cup. However, this method is slow and extremely expensive. A longer runner system is needed which causes excessive waste. Of course, one could reduce the waste by using a heated runner system that keeps the plastic in a molten condition. But, heated runner systems are expensive and may easily burn soft plastic, leaving dirt in the cups.